The present invention relates to a data transmission system and, more particularly, to a data transmission system which has a transmission line to which a plurality of node stations are connected in parallel and which increases the effective transmission speed.
In data transmission where a plurality of node stations are connected in parallel to the transmission line, (a multidrop connection), when node stations transmit data at random, a plurality of signals can simultaneously appear on the same transmission line. Thus, proper transmission often cannot be performed. The interference of signals from at least two signal sources on the same transmission line is called a contention. In order to prevent the contention and to smoothly exchange data between the node stations, a set of given rules is introduced for a data link to control the transmission. The set of given rules is called a protocol. Various types of protocols are used in data transmission or communication.
In these protocols, to maintain an orderly data link, the control function for the data link is given to one of the node stations. A station which performs the control function for a data link is called a master station. A station is defined as an independently controllable configuration of data terminal equipment from or to which messages are transmitted on a data link. The master station first indicates the start of transmission and then transmits data to a given slave station or receives data therefrom. The data link may have a configuration wherein the master station is predetermined and the remaining stations are defined as the slave stations. Alternatively, the data link may have another configuration wherein any station can be defined as the master station and the remaining stations are defined as the slave stations. The former configuration is called a 1:N communications system (master station:slave stations; 1:N). The latter configuration is called an N:N communications system. The present invention is concerned with the N:N communications system in which the master control function can be assigned to any station.
FIG. 1 shows a model of a conventional contention data transmission system.
Stations A and B respectively designated by reference numerals 12 and 14 are connected in a multi-drop manner to a transmission line 10. A busy status line 16 is used to indicate the occupying status of the transmission line 10. An output from an open collector transistor of and a TTL input gate of each station is connected to the busy status line 16. When a given station occupies the transmission line, the transistor is rendered ON. The ON/OFF state of the input gate determines whether or not the transmission line is occupied (busy). The station which initiates transmission sets the busy status line 16 to ON so as to indicate to other stations that the transmission line is busy. In this manner, when the busy status line is ON, other stations do not perform transmission. After completion of transmission, the master station sets the busy status line to OFF and returns to a slave station.
FIGS. 2A to 2D are timing charts showing the occupying status of the stations A and B and the status of the busy status line 16 in the data link in a time serial manner. In time intervals P1 and P2 of FIG. 2D, since one of the stations A and B is a master station while the other is not busy, no problem occurs. However, in time interval P3, since the transmission line 10 is not used, the stations A and B may simultaneously transmit data onto the transmission line 10. In this case, a contention occurs. In order to resolve this situation, all the stations stop transmitting data. Thereafter, retransmission is started, with a priority given among those stations so as not to cause a contention again. Time interval P4 indicates a time interval for resolving the contention in the transmission line. Referring to FIGS. 2A and 2B, the station A has a priority over the station B.
The contention system has an advantage in that a relatively simple protocol can be used for the data link, so that simple software can be used for a microprocessor. However, the following drawbacks are presented when high-speed and high-efficient data transmission between the stations is performed.
(1) When a small number of stations constitute a network and data is not frequently transmitted, the possibility of contention occurrence is low. However, when a large number of stations are used and the N:N transmission is performed, the possibility of contention occurrence becomes high. The data link can be thus locked for a long period of time.
(2) When many stations constitute the network, retransmission restoring procedures at the time of a contention occurrence become complex.
(3) Since a station which has the lowest priority has a small possibility of transmission line occupation, the overall efficiency of the data link is decreased.
The polling/selecting system will now be described. According to this system, the master control function is handed like a baton pass among the stations by polling and selecting without causing a contention. At this time, the transmission line is controlled by the master station. Therefore, a busy status line need not be used which indicates the status of the transmission line. The N:N communication data link using the polling/selecting system has the following advantages:
(1) Unlike the 1:N communications system, even if the master station is broken, the data link may not be disabled as a whole.
(2) It is possible to exchange data between two arbitrary stations of the data link.
(3) If the master control function of the current master station is assigned to another station which has the next number to that of the current master station, the transmission line occupation possibility of each station becomes substantially equal, thus eliminating the drawback of the contention system where the master control function is not equally assigned to each station.
(4) Since the master control function is assigned to a station in accordance with a given set of rules, the time interval required for restoring the transmission status after the contention occurrence is eliminated, resulting in high efficiency of use of the transmission line.
However, a problem occurs when the data link of the polling/selecting system is applied to a decentralized total control system. The decentralized type total control system such as a direct digital control measuring system comprises process control stations for exchanging required control data at high speed and a supervising station for connecting the control data (e.g., set value, control value, and processing value) from the process control stations. In this system, the overall efficiency of hierarchical network node stations and their effective transmission speed are significant.
When the data link of the polling/selecting system is used, the following problems are presented.
(1) In the case where the master station searches a station intended to be assigned the master control function, no problem occurs when the master station immediately finds such a station. Otherwise, the master station continues to search such a station until it is found. Thus, a long search time interval is required, prolonging the control period. The more significant control procedure is influenced. In order to eliminate the above drawback, a master search is interrupted for a given period of time if no station is found which can be assigned the master control function. In this case, special software such as a timer operation is required, thus leaving room for further improvement.
(2) In a case where the supervising station collects data, typical data of the control loop is periodically collected except for cases of remote control by the operator and of emergency analysis. When the time for data collection is reached, the master control function is assigned to the supervising station which then controls the slave stations to collect data. However, if the priority of assignment of the master control function to the supervising station is equivalent to that of the process control station for controlling the processings, it takes a long time interval for the supervising station to acquire the master control function. As a result, the change period of the control variable of the CRT display loop is prolonged.
(3) The time interval required for collecting data from the slave stations by the master station can be determined from FIG. 3. Vertical arrows indicate operating time intervals of the microprocessors in the master or slave stations and horizontal arrows indicate operating time intervals required for transmitting data between the two stations. These operating time intervals vary with the amount of data. It is noted that the length of the arrow is not in proportion to the actual time interval. Reference symbols t1 and t5 respectively denote transmission operating time intervals of the master and slave stations. In these time intervals, data are set to a transmitter LSI and a DMA control LSI. Reference symbols t2 and t6 respectively denote time intervals for transmitting data or a command on the transmission line, which vary with the transmission speed of the transmission line and the amount of data (number of bytes) to be transmitted. In serial transmission, the transmission time is calculated by the relation: t=(n.times.8)/m (ms) where m is the transmission speed (kilobit/sec) and n is the amount of data (number of bytes). Therefore, the faster the transmission speed is, the faster data can be transmitted. Reference numerals t3 and t7 denote time intervals in which the microprocessor receives an interrupt signal indicating completion of reception of the command or data. Since the interruption may be masked for the software operation of the microprocessor and the higher priority order of interrupt signals must first be detected, the reception of the interrupt signal cannot be immediately confirmed by the microprocessor. Thus, the time intervals t3 and t7 cannot be neglected. Reference symbols t4 and t8 denote operating time intervals for which the microprocessor operates based on the interrupt signals indicating the completion of the data or command transmission. A given slave station checks whether or not a reception command error occurs and which command is received. Specified data is then written at a predetermined address of the memory. Thus, the given slave station prepares data to be transmitted to the master station in response to the command. The master station checks whether external noise is mixed in with the data response from the given slave station and whether requested data is returned. The master station then stores data in another memory area. The master station repeats the above operation for another given slave station.
Time interval T required for collecting data from a single slave station is equal to the sum of the time intervals t1 to t8: ##EQU1##
It takes at least the time interval NT for the master station to collect data from N slave stations. As a result, the processing speed of the system as a whole cannot be increased.
(4) The next significant problem is a matter of time matching of the data of each loop control station from which the supervising station collects data. As described in item (3), when data from the first slave station of N slave stations is compared with that from the Nth slave station thereof, data of the latter is lagged by time interval (N-1)T. In this manner, pieces of data from different slave stations are slightly lagged. No problem occurs when this time lag can be neglected. However, if the number of slave stations is increased, the amount of data is also increased. If the data transmission time interval is increased, simultaneity of the supervision is apparently affected.
The above problems of items (1) to (4) must be solved to accomplish a high-speed and precisely controllable diffusion type total control system.